1. Field of the Invention
This invention pertains generally to fabrication methods and applications of nanowire-polymer composite electrodes, and more particularly to methods for fabricating flexible, transparent conductors utilizing conductive nanowires with a smooth active surface for use in electronic devices such as light emitting devices and solar cells.
2. Description of Related Art
Organic electronic devices such as organic light emitting diodes (OLEDs) and flexible OLEDs have advanced tremendously in recent years. These devices can be fabricated on to large areas and substrates and are compatible with low-cost fabrication processes. Such flexible and deformable electronic devices are needed for many new applications such as wearable displays, solar panels, touch screens, electro-chromic devices and non-invasive biomedical devices, where large deformations may be required to cope with body movements. Deformable devices could also enable conformation onto irregular mounting surfaces.
Electrodes for such electro-optical devices normally must be transparent and flexible. Such flexible electrodes require high transparency and conductivity, strong adhesion, flexibility of the conductive layer with respect to the substrate as well as a smooth conductive layer surface. High conductivity with transparency characteristics are required to avoid undesirable voltage drops and the Joule heating effect in the device, especially for current based devices such as organic light emitting diodes and solar cells. Both the flexibility of the conductive layer and strong adhesion to the substrate are required to keep the conductive layer from delamination. Surface smoothness is also important because the thickness of active layer in these electro-optical devices is typically in the range of tens of nanometers. Any spikes on the electrode layer can cause an undesirable short circuit that leads to device failure.
Early flexible OLED attempts used indium-doped tin oxide (ITO) as the conductive layer in transparent electrodes. However, device flexibility and performance are quite limited in these electrode constructs. Various other transparent electrodes have been investigated in order to replace ITO electrodes to enhance the flexibility of the thin film devices.
Vapor deposition methods such as plasma vapor deposition (PVD) or chemical deposition (CVD) are conventional process that could be used to produce flexible transparent electrodes. However, vapor deposition schemes require complicated equipment and a huge capital investment, greatly increasing the cost of the device fabrication process. In addition, the materials available for use in such conventional vapor deposition processes are limited to mainly metal oxides or mixed metal oxides, such as indium-tin mixed oxide (ITO), antimony-tin mixed oxide (ATO), fluorine-doped tin oxide (FTO), and aluminum-doped zinc oxide (Al—ZO). Although electrodes produced from these materials may be both transparent and moderately conductive, metal oxides are found to be brittle and the adhesion to the substrate is poor. Such electrodes do not survive in applications that require repeating deformations.
Another approach is a sol-gel process for applying metal oxides or mixed metal oxides by hydrolysis and partial condensation of a precursor to form a stable sol followed by a normal coating process. The wet coating undergoes a thermal curing process to form a continuous polymeric metal oxide or mixed metal oxide network. However, coatings formed by sol-gel process typically have a loose and porous structure and have low conductivity. Densification of the coating requires an annealing process at temperatures much higher than the flexible substrate can endure. This process is therefore unsatisfactory.
In a further possible method, conductive nano-particles, nanotubes, nano-sheets, or nano-wires may be dispersed in a solution to form a stable formulation that is then applied to a flexible substrate by a normal coating method, such as spray, dip, flow, slot coating, and Meyer rod methods. However, nano-materials produced by these methods are not stable in solution and tend to form agglomerations after initial dispersion. Therefore, a dispersant must be used to stabilize the dispersion. Typical dispersants are nonconductive and must be removed during the coating process or shortly afterwards to maintain the conductivity of the coating. Coatings often delaminate from the substrate during the washing process. Decomposing and burning off of the dispersant from the coating is not an option because the flexible substrate cannot endure extremely high temperatures.
Coatings formed on a substrate by conventional spray, dip, or Meyer rod methods also suffer large surface-height variations, typically well above 100 nm, which can often be greater than the thickness of active layer in a typical electro-optical device. These spikes on the electrode layer can cause undesirable short circuits that lead to device failure.
Accordingly there is a need for flexible transparent electrodes with excellent optical transparency, electrical conductivity, conductive layer flexibility and strong adhesion to the substrate and active surface smoothness. A need also exists for a method which renders a low cost, scalable process for producing a flexible transparent electrode for use in optical-electrical devices. The present invention satisfies these needs, as well as others, and is generally an improvement over the art.